Tunnelling under Niagara Falls sounds like a daunting task, but since September 2006 the world’s largest hard rock tunnel boring machine has been doing just that. The Robbins 14.4m Main Beam TBM is boring a 10.4km long hydroelectric tunnel for the Niagara Tunnel project in Queenston, Ontario, Canada. The tunnel will connect to the Sir Adam Beck Power Station, owned by Ontario Power Generation (OPG). The new tunnel will add to the capacity of the power station by supplying 500m3 of water per second. Annually, the 150MW generated by the new tunnel will be enough to supply a city of 700,000 people.

The TBM, back-up system, and conveyor system were all supplied by separate companies and are specially designed for simultaneous tunnelling and lining operations in order to meet an aggressive schedule. The TBM was assembled onsite, rather than in the Robbins manufacturing facility, saving approximately 4-5 months on the TBM delivery schedule. Several critical components were manufactured and pre-assembled in the workshop to insure fit-up before being transferred to the site, where the majority of the machine was assembled using operating personnel. Robbins supplied a specialised supervisory and labour team, while contractor strabag AG supplied local labour. The entire time from TBM order to machine launch spanned less than 12 months.

Time will be saved on the overall project schedule in other ways as well. The 12.5m finished diameter tunnel will require 50cm thick concrete lining with a waterproof membrane to prevent water from leaking out of the tunnel. As the TBM bores, the tunnel will be concurrently lined with in-situ concrete and PVC waterproofing membrane.

The invert structure will be cast and set approximately 500m behind the TBM boring operations, while the arch structure will be separately cast approximately 1500m behind the machine. An 87m long bridge will allow rubber-tired supply vehicles to travel over the invert concrete installation area. While the arch is cast, the ventilation duct, continuous conveyor, supply pipes and power lines will need to be temporarily removed from the bored tunnel walls and diverted through the concrete formwork until they can be reattached to the completed tunnel walls further down.

The concurrent lining works will be started once the TBM has bored ahead 1km – scheduled to occur in late May. ‘The installation of the in-situ concrete liner continuous while boring is a first in North America and was initiated by Strabag in order to reduce the construction schedule and reduce the cost of the tunnel as compared to pre-cast segments, ‘ says Doug Harding, Vice President of The Robbins Company-Solon.

Other simultaneous operations include a mono-rail crane system attached to the tunnel crown that operates independently of the TBM and allows the rail to be moved forward as the TBM advances. The rail, in 4.5m sections, is removed from behind the bored section of tunnel and transported over the back-up to a new section, leaving behind a smooth tunnel floor. ‘The simultaneous operations result in less rolling stock and materials that must be maintained. The rail does not have to be removed as a separate step after the tunnel bore is complete,’ says Mike Burngasser, Robbins Field Service Manager.

Larger diameter 20-inch back-loading cutters are another innovation being used on the TBM to bore more efficiently. The cutters have 20-25% more wear material than their 19-inch diameter counterparts, resulting in a longer cutter life, fewer cutter changes, and ultimately less TBM downtime.

As of late April 2007, the TBM had bored ahead approximately 730m in varied geology consisting of limestone, dolostone, sandstone, shale and mudstone in a -7.28% incline. The machine is expected to bore through Queenston Shale for the majority of the drive. The bore path includes an initial steep downgrade, as well as a steep upgrade at the intake, in order to avoid a 100m deep buried gorge. Horizontal realignment of the tunnel was not possible due to existing water diversion tunnels already in place.

The rock so far encountered, up to 180 MPa UCS, has been largely competent with some minor broken ground. A specially designed foam system has also helped increase the TBM performance in sticky ground. The water spray normally applied to the cutterhead has been temporarily replaced with the foam system to assist in the flow of the material through the cutterhead. Five openings in the cutterhead allow the foam to be plumbed in, where it mixes with water and air. The ground is expected to further improve once the TBM hits the 1km mark, where shale should be predominant.

Shotcrete is being used throughout the drive as the primary means of tunnel support. Rubber-tired tractors transport shotcrete from the onsite batching plant to two shotcrete robots located on the back-up system. Each robot has 360-degree coverage and can travel up to 8m in the longitudinal direction to spray shotcrete at the rate of 20m per hour. Additional types of rock support include ring beams, wire mesh, and rock bolts.

The tunnelling crew operates the TBM and equipment 24 hours a day, seven days a week. About 30 crew members are onsite in a given shift, with one maintenance shift each morning in order to monitor and test the equipment. The crew has endured winter conditions reaching below -20°C at the jobsite, which caused the conveyor systems to freeze over with ice. Antifreeze was sprayed on the affected conveyors and the ice was chipped off in order to keep them running. The continuous conveyor system is being installed as the back-up advances, and will transport 1.7Mm3 of muck over three years to a storage area on OPG property.

The 51-month design-build turnkey contract for Austria-based Strabag AG calls for the tunnel to be operational by December 2009, and for tunnel boring to be completed by September 2008.

The 14.4m diameter Niagara TBM was launched in September 2006 and had bored 730m as of April 2007. Photograph courtesy of Ontario Power Generation Niagara TBM The Robbins machine is the world’s largest hard rock TBM. Photograph courtesy of Ontario Power Generation Largest TBM 50cm thick in-situ concrete invert segments will be cast 500m behind the Robbins TBM. Photograph courtesy of Ontario Power Generation Segments Winter conditions at the Niagara jobsite reached below -20 degrees Celsius and required the use of antifreeze on the conveyor system Winter The back-up features a variety of tunnel support equipment, including two shotcrete robots that can spray shotcrete at a rate of 20m per hour Robots The Niagara TBM is the first machine ever to use 20 inch diameter back-loading cutters, which increase cutter life Cutters Author Info:

For further information on the Niagara Tunnel project visit www.opg.com or www.strabag.com. For details on the TBM visit www.TheRobbinsCompany.com